This behavior is expected and the correct way of functioning. When a modal RadWindow is opened, all the content below the modal overlay should be inaccessible and when a RadWindow is used with ContentTemplate, its content resides on the same page. As result, the first RadWindow takes all the elements on the page including the ones in the second window and sets their tabindex to -1 to make them inaccessible. The original tabindexes are restored when the window is closed.
To get the desired scenario and result, I can suggest the following approaches:
1. You could use a RadWindow with a specified NavigateUrl
for the second window - in this manner the first one will not access its content elements - this is the better approach in case there are too many elements with tabindexes.
All the best,
the Telerik team
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